An arrhythmia is a medical condition in which there exists a problem with the rate or rhythm of the heartbeat usually due to abnormal electrical activity in the heart. More specific types of arrhythmia include when the heart beats too fast (known as tachycardia), too slow (known as bradycardia) or with an irregular rhythm (known as cardiac fibrillation). Two general devices are known in the art for helping people who experience arrhythmias. One is known as a pacemaker, the other is known as an implantable cardioverter defibrillator (herein abbreviated ICD). Pacemakers are implantable devices which continuously measure the heartbeat and electrical activity in the heart. Pacemakers can detect irregularities in the heartbeat, i.e. arrhythmias, and are programmed to provide electrical signals to the heart to restore its normal beating rhythm.
Reference is now made to FIG. 1A, which is a schematic illustration of a pacemaker or ICD with intravascular leads implanted in a patient, generally referenced 10, as is known in the art. As shown in FIG. 1A, a pacemaker 12 is implanted in a patient 14, having a heart 16 and a ribcage 18. Pacemaker 12 includes two main components, a can 20 and electrical leads 22. Can 20 includes a power source (not shown), such as a battery, as well as an electronic circuit (not shown) for monitoring the electrical activity in the heart and for providing electrical signals to the heart when aberrant rhythms of the heart are detected. The electronic circuit may include at least one low voltage capacitor (not shown). Can 20 is usually implanted in patient 14 via a surgical procedure on his left side adjacent to and below the clavicle bone (also known as the collarbone), as shown by an arrow 24 in FIG. 1A. Electrical leads 22 are coupled with the electronic circuit in can 20 at one end and are coupled intravascularly with heart 16 at the other end, the electrical leads being inserted through the subclavian vein (not shown) and the vena cava (not shown). Electrical leads 22 are typically implanted in patient 14 by inserting them percutaneously through his vena cava (not shown). Once attached to heart 16, they are coupled with can 20. Electrical leads 22 are usually flexible and provide electrical signals of heart 16 to the electronic circuit in can 20 as well as providing electrical signals from the electronic circuit to heart 16. Typically, electrical leads 22 are implanted in right ventricle 26 and right atrium 28 of heart 16.
ICDs are similar to pacemakers and include similar components, such as a can and electrical leads; thus pacemaker 12 in FIG. 1A could also be an ICD. An ICD differs slightly from a pacemaker in that its can includes a power source, electronics, electrical leads as well as at least one high voltage capacitor. The electronics of an ICD includes a sensing algorithm to detect ventricular fibrillation, a functionality not included in standard pacemakers. The difference between an ICD and a pacemaker is that an ICD can deliver a high voltage electric shock to the heart to terminate an otherwise potentially fatal cardiac tachyarrhythmia. A pacemaker is generally limited to treating bradyarrhythmias which can be treated with a significantly lower voltage electric impulse. The presence of at least one high voltage capacitor in an ICD accounts for its difference in function from a pacemaker as the at least one high voltage capacitor enables a significantly higher electrical shock to be built up and delivered to the heart. An additional function of an ICD is to send the heart an electrical shock in case of ventricular fibrillation (herein abbreviated VF) and in order to prevent cardiac arrest, i.e., aborted sudden death. The electrical energy required for the electrical shock is built up and stored in the at least one high voltage capacitor. ICDs exist as standalone devices yet are also manufactured having the functionality of a pacemaker. In addition, cardiac resynchronization therapy defibrillators (herein abbreviated as CRT-D), which are a type of ICD, include a third electrode allowing for simultaneous pacing of both the right and left ventricles of the heart.
As mentioned above, ICDs, similar to pacemakers, constantly monitor the rate and rhythm of the heart and deliver therapies to the heart by way of an electrical shock. In the case of an ICD, electrical shocks are provided to the heart when the measured electrical activity of the heart exceeds a preset number. State of the art ICDs can distinguish different types of aberrant electrical activity in the heart, such as VF, when the heart contracts irregularly, versus ventricular tachycardia (herein abbreviated VT), when the heart beats regularly but significantly faster than normal. In the case of VT, such ICDs may send electrical signals to the heart to try and pace the heart faster than its intrinsic heart rate in an attempt to stop the tachycardia before it progresses to VF. This technique is known in the art as fast-pacing, overdrive pacing or anti-tachycardia pacing (herein abbreviated ATP). As is known to workers skilled in the art, ATP is only effective if the underlying rhythm of the heart is ventricular tachycardia. ATP is never effective if the heart is already experiencing ventricular fibrillation and thus lacks a consistent heart rate. State of the art ICDs use a combination of various methods to determine if received electrical signals from the electrical leads represent a normal rhythm of the heart, ventricular tachycardia or ventricular fibrillation. It is noted that the placement of an ICD in the body of a patient is similar to that of a pacemaker, however in the case of a CRT-D device, the electrical leads can also be implanted in the left side of the heart via the coronary sinus (not shown) of the heart. This is shown in FIG. 1A as an electrical lead 30, denoted by a dashed line. Pacemakers and ICDs with intravascular leads are known in the art. As an example, U.S. Pat. No. 5,133,353 to Hauser, assigned to Cardiac Pacemakers, Inc., entitled “Implantable intravenous cardiac stimulation system with pulse generator housing serving as optional additional electrode” is directed to an implantable cardiac stimulation lead system having pacemaking, cardioversion and higher energy defibrillation capabilities. The implantable cardiac stimulation lead system also has a selectable electrode configuration and utilizes a relatively small number of implantable parts. The lead system comprises a transvenous myocardial, or pericardial lead having a plurality of electrodes as well as pulse generator circuitry. The lead electrodes are capable of sensing and performing standard anti-bradycardia pacing, anti-tachycardia pacing, cardioversion and defibrillation. The transvenous lead is connected to a pulse generator having full-function pacing capabilities as well as cardioversion and defibrillation capabilities. The housing of the pulse generator is conductive and is connected to the pulse generator circuitry so that it may selectively serve as a discharge electrode. The outer surface of the pulse generator could be of a special configuration to facilitate its discharge capabilities. The pulse generator is implanted in the pectoral or abdominal region of the body proximate the heart. A programmable switch or other type of circuitry is provided to select the electrode configuration which may include or exclude the pulse generator housing electrode. As a result, different electrode configurations can be obtained for specific types of cardiac stimulations. Other examples of such heart devices with intravascular leads include U.S. Pat. No. 5,261,400 and WO 2003/002198 (both to Medtronic, Inc.), WO 2004/028628 (St. Jude Medical), U.S. Pat. No. 6,256,541 (Cardiac Pacemakers, Inc.), US 2012/0165913 A1 and EP 1 631 350 B1 (proprietor Cameron Health Inc.).
Known in the art as well are intravascular ICDs, also known as percutaneous ICDs, in which the entire device, including all the components found in a can and the leads, is positioned within the vasculature of a patient. As an example, U.S. Pat. No. 7,899,554 B2 to Williams et al., assigned to Synecor LLC, entitled “Intravascular system and method” is directed to an intravascular implantable pacing and/or defibrillation system. The system includes a pulse generator that is implantable within a blood vessel and at least one electrode attachable to the pulse generator. The pulse generator is proportioned to blood flow through the blood vessel. During implantation, the pulse generator is introduced into a patient's vasculature, advanced to a desired vessel and anchored in place within the vessel. The electrode or electrodes are placed within the heart or surrounding vessels as needed to deliver electrical pulses to the appropriate location. Other examples of such intravascular ICDs are described in U.S. Pat. No. 7,617,007 B2 and U.S. Pat. No. 8,311,633 B2 (all assigned to Synecor LLC). These intravascular ICDs however are not yet available in the market.
Pacemakers and ICDs with intravascular leads, as shown in FIG. 1A, are advantageous in that the electrical leads used for sensing arrhythmias as well as delivering electrical shocks and impulses to the heart are placed directly in the heart (i.e., hence intravascularly). Such a placement of the electrical leads allows for a significantly high signal-to-noise ratio (herein abbreviated SNR) such that aberrant electrical activity detected in the heart is in fact aberrant electrical activity of the heart and not electrical activity coming from another source of electrical activity in the body near the heart or from a source outside the body generating an electric field. Also, the closeness of the electrical leads to the chambers of the heart enables a generally lower voltage to be applied to the heart for either pacing it or for treating VT or VF via electrical shocks. Such pacemakers and ICDs however are disadvantageous in that major surgery is required to implant the can in the body and the electrical leads in the vasculature of the heart. This disadvantage is true of intravascular ICDs as well as the entire device must be implanted in the vasculature of the patient. Furthermore, when the energy of the battery is depleted, or if there is a problem with the electrical leads placed in the heart, the patient must undergo further surgery to either replace the entire can or to have new electrical leads placed in the heart. Pacemakers and ICDs having cans with replaceable and/or rechargeable batteries are currently not on the market, thus when the battery of such devices is depleted, the entire can of the device (pacemaker or ICD) must be replaced.
In the past decade, there has been a general trend in surgery and implantable medical devices to reduce the amount of invasiveness of either the surgery involved or the positioning of the implantable medical device in the body of a patient. For example, in the field of ICDs, medical device companies have begun researching and developing subcutaneous ICDs which are to be placed under the skin and around the heart, thereby significantly reducing the invasiveness of an implanting procedure and the actual positioning of the ICD in the body of the patient. One of the reasons for this trend in ICDs is that many health-related issues have occurred with the intravascular and intracardiac leads used in prior art ICDs, including the recall of such leads. Intravascular and intracardiac leads move a tremendous amount within the heart as it beats during the lifespan of a prior art ICD. With an average of 60 movements per minute over the course of seven years, an intravascular lead may move over 220 million times. These leads thus require a very high durability due to the continuous movement of these leads within the heart and can wear and break over time, causing serious problems to the patient, including patient death. Major companies in this field include Boston Scientific, Cameron Health (acquired by Boston Scientific), Medtronic and St. Jude Medical. Of these companies, only Cameron Health has an actual subcutaneous ICD device in the market.
Reference is now made to FIG. 1B, which is a schematic illustration of a subcutaneous ICD implanted in a patient, generally referenced 40, as is known in the art. A patient 44 is shown, having a heart 46 and a ribcage 48. A subcutaneous ICD 42 in placed under the skin near the heart. Subcutaneous ICD 42 includes a can 50 and electrical leads 52, each respectively similar to can 20 (FIG. 1A) and electrical leads 22 (FIG. 1A). Can 50 can also be referred to as a canister. Can 50 is usually positioned under the skin around a fifth left rib 51, near the heart (i.e., laterally to the heart), whereas electrical leads 52 are positioned around heart 46. Usually a first electrical lead is positioned anterior to heart whereas a second electrical lead is positioned posterior to heart, thus creating an electrical shock vector between the two electrical leads via heart 46. Subcutaneous ICD 42 thus also has a can and leads configuration, similar to pacemaker 10 (FIG. 1A). Subcutaneous ICDs having a can and leads configuration are known in the art. As an example, U.S. Pat. No. 6,721,597 B1 to Bardy et al., assigned to Cameron Health, Inc., entitled “Subcutaneous only implantable cardioverter defibrillator and optional pacer” is directed to a subcutaneous implantable cardioverter-defibrillator (S-ICD) having an electrically active canister which houses a battery supply, capacitor and operational circuitry where the canister serves as an electrode and replaces one conventional lead of a traditional system. The canister also has one or more subcutaneous combined high voltage/sense/pace electrodes and sense circuitry suitable to an ICD or AED V-FIB detection algorithm. The S-ICD further has an application system for simple insertion of the subcutaneous lead and a cutaneous test system designed to estimate the best location of the S-ICD for each patient. Cardioversion-defibrillation energy is delivered when the operational circuitry senses a potentially fatal heart rhythm. There are no transvenous, intracardiac or epicardial electrodes used in the S-ICD. Other examples include the following patents and patent applications: U.S. Pat. No. 8,483,841 B2, U.S. Pat. No. 8,644,926 B2 (all assigned to Cameron Health Inc.), U.S. Pat. No. 8,260,415 B2, U.S. Pat. No. 8,512,254 B2, U.S. Pat. No. 8,359,094 B2, U.S. Pat. No. 7,894,894 B2 (all assigned to Medtronic Inc.) and EP 2 510 973 A1 (applicant Cardiac Pacemakers Inc.).
Subcutaneous ICD 42 is advantageous over an ICD with intravascular leads and an intravascular ICD in that major surgery is not involved in its placement and improved safety is provided to the patient since the insertion of the electrical leads of the ICD does not involve any intervention with the heart or puncturing of a blood vessel. Replacing can 50 or replacing electrical leads 52 if they are faulty is also simpler in that only percutaneous surgery is involved. However, since subcutaneous ICD 42 and its electrical leads are not placed in the vasculature of the heart, electrical leads 52 may have a significantly lower SNR and thus the electric circuit (not shown) in can 50 may have a harder time differentiating between electrical activity of the heart and what is known in the field as extra-cardiac oversensing or extra-cardiac noise (i.e., electrical activity sensed from non-cardiac muscles around the heart and electrical activity coming from sources outside the patient). This difficulty in differentiating between true electrical activity of the heart and extra-cardiac oversensing can lead to subcutaneous ICD 42 delivering shocks to the heart when it doesn't need it and also failing to deliver shocks to the heart when it does need it. In addition, since electrical leads 52 are not placed directly in heart 46, a higher voltage must be applied to the leads for treating VT or VF via electrical shocks as compared with conventional ICDs (as in FIG. 1A) in which its leads are placed intravascularly directly in the heart. The higher voltage requires a higher level of energy. The higher level of energy thus requires a larger can volume since the can requires a larger battery and larger high voltage capacitors to provide the higher energy requirements. The can and leads configuration of subcutaneous ICD 42 may also cause discomfort to patient 44, especially considering that the rigid outer surface of can 50 is placed directly on ribcage 50 where humans in general do not have a lot of excess skin or fat tissue in this particular region of the body to cushion can 50. A further disadvantage of a subcutaneous ICD is that due to its placement in a patient, many sensory and motor nerves are located between the electrical leads. Any stimulation generated between the electrical leads for the heart will be felt by the patient as both muscle contractions (i.e., from the motor nerves) and pain (i.e., from the sensory nerves). This is much less of a concern for an ICD with intracardiac leads, especially when stimulation is generated between the leads in the heart, as the electric field generated is essentially limited to the area of the heart and does not cause muscle contractions or the sensation of pain around the heart. If it for this reason that subcutaneous ICDs generally do not provide a pacing function.
Some of the concerns with subcutaneous ICD 42 have been mitigated by medical device companies using a different configuration for subcutaneous ICDs, such as a curved configuration. Reference is now made to FIG. 1C, which is a schematic illustration of another subcutaneous ICD implanted in a patient, generally referenced 70, as is known in the art. A patient 74 is shown having a heart 76 and a ribcage 78. A subcutaneous ICD 72 in placed under the skin near the heart. Subcutaneous ICD 72 includes a housing 73. Housing 73 includes a plurality of surface electrodes 80, an electric circuit (not shown), a battery (not shown) and at least one high voltage capacitor (not shown), similar to the elements found in subcutaneous ICD 42 (FIG. 1B). Housing 73 has a curved configuration, being thin, narrow and flexible, similar to a patch, bandage or plaster and shaped to fit around a patient's rib. Plurality of surface electrodes 80 are positioned on one side of housing 73, giving subcutaneous ICD 72 a specific directionality. As shown in FIG. 1C, a first surface electrode 82A and a second surface electrode 82B are placed on an inner side of housing 73, facing towards the body (not labeled) of patient 74. As compared with subcutaneous ICD 42, subcutaneous ICD 72 does not have any electrical leads. Instead first surface electrode 82A and second surface electrode 82B are used to both sense electrical activity of heart 76 as well as apply electrical shocks to heart 76. Plurality of surface electrodes 80 thus function as electrical leads.
Housing 73 is usually positioned under the skin around a fifth left rib 84, near the heart. Since housing 73 is flexible, it is usually wrapped around fifth left rib 84, or near it, following the contours of ribcage 78 and partially wrapping around heart 76. A proximal end (not labeled) of housing 73 may be anterior to heart 76 and a distal end (not labeled) of housing 73 may be posterior to heart 76. An electrical shock vector is thus created between plurality of surface electrodes 80 via heart 76. It is noted that housing 73 is usually made of metal and can also function as a sensor or electrical lead. Housing 73 is thus also referred to in the art as an active can. In such a configuration, one of the surface electrodes can be used to sense electrical activity whereas the other surface electrode can be used with housing 73 to create an electrical shock vector. Subcutaneous ICDs having a curved configuration are known in the art. As an example, U.S. Pat. No. 6,647,292 B1 to Bardy et al., assigned to Cameron Health, entitled “Unitary subcutaneous only implantable cardioverter-defibrillator and optional pacer” is directed to a unitary subcutaneous implantable cardioverter-defibrillator having a long thin housing in the shape of a patient's rib. The housing contains a source of electrical energy, a capacitor and operational circuitry that senses the presence of potentially fatal heart rhythms. Provided on the housing are cardioversion/defibrillation electrodes located to deliver electrical cardioversion-defibrillation energy when the operational circuitry senses a potentially fatal heart rhythm. The unitary subcutaneous implantable cardioverter-defibrillator does not have a transvenous, intracardiac, epicardial or subcutaneous electrode. Other examples include the following patents: U.S. Pat. No. 7,363,083 B2, U.S. Pat. No. 8,718,760 B2 (all assigned to Cameron Health Inc.) and U.S. Pat. No. 7,684,864 B2 (assigned to Medtronic Inc.).
Whereas subcutaneous ICD 72 may be more comfortable for a patient than subcutaneous pacemaker 42 (FIG. 1B) due to its flexible thin shape and slightly reduced invasiveness since only a single element needs to be implanted in patient 74, surgery is still required to replace a dead battery in subcutaneous ICD 72. In addition, subcutaneous ICD 72 may suffer the same SNR issues that accompany subcutaneous ICD 42 in terms of differentiating true cardiac electrical activity compared to extra-cardiac oversensing. In addition, as mentioned above subcutaneous ICD 72 has a particular directionality and must be placed in a specific orientation to function properly in patient 74.